Apparatus for scanning an underwater area

ABSTRACT

The apparatus of this invention is designed to be generally operated from either a towed or self-propelled underwater vehicle. A nearly collimated beam of light from a rotating cylinder traces a narrow strip of the target area during each rotation. The strip is substantially perpendicular to the direction of travel of the apparatus. During subsequent rotations, a series of parallel strips are traced on the target area. The speed of rotation and corresponding raster scan on the display monitor is synchronized to the forward speed of the vehicle to which the apparatus is attached. The instantaneous field of view of the receiving optics is stopped down so that only the light from a small area at the center of the intersection of the collimated source beam and the target area is viewed. The size of this area corresponds to the minimum resolution element of the device. The location of the field stop of the receiving optics is a function of the viewing range, and is adjusted accordingly. The automatic gain control on the output signal from the photomultiplier tube compensates for the varying propagation losses due to varying path lengths in the mapping geometry. The intensity displayed at a particular spot on the video monitor or stored at a given location on the video tape is therefore a map of the optical reflectivity of the corresponding spot on the surface of the target area.

United States Patent [191 Funk et al.

[ APPARATUS FOR SCANNING AN UNDERWATER AREA [75] Inventors: Clarence J.Funk; Ivor P. Lemaire;

Jerry L. Sutton; Frederick A. Marrone, all of San Diego, Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

[22] Filed: May 1, 1972 [2]] Appl. No.: 249,059

[52] US. Cl. 340/4 R, l78/6.6 R, 350/6, 356/4 [51] int. Cl. G01] l/ [58]Field of Search 340/4 R; 356/4, 5; 350/6, 7; 343/5 CM, 5 PC; l78/6.6 R

[56] References Cited UNITED STATES PATENTS 3,533,697 lO/l970 Hughes356/5 Primary Examiner-Richard A. Farley AtlorneyRichard S. Sciascia etal.

[5 7] ABSTRACT The apparatus of this invention is designed to be gen-[451 Nov. 27, 1973 erally operated from either a towed or self-propelledunderwater vehicle. A nearly collimated beam of light from a rotatingcylinder traces a narrow strip of the target area during each rotation.The strip is substantially perpendicular to the direction of travel ofthe apparatus. During subsequent rotations, a series of parallel stripsare traced on the target area. The speed of rotation and correspondingraster scan on the display monitor is synchronized to the forward speedof the vehicle to which the apparatus is attached. The instantaneousfield of view of the receiving optics is stopped down so that only thelight from a small area at the center of the intersection of thecollimated source beam and the target area is viewed. The size of thisarea corresponds to the minimum resolution element of the device. Thelocation of the field stop of the receiving optics is a function of theviewing range, and is adjusted accordingly. The automatic gain controlon the output signal from the photomultiplier tube compensates for thevarying propagation losses due to varying path lengths in the mappinggeometry. The intensity displayed at a particular spot on the videomonitor or stored at a given location on the video tape is therefore amap of the optical reflectivity of the corresponding spot on the surfaceof the target area.

Claims, 4 Drawing Figures 12W jf/fousw 2 A, A V n 2 va/vz/rPagr fi amwaaT ME 5232- I/WED an/ER ,L/ -l 116 2 FIFE fu/my fuzz/E 706E at V T. L [V(I 26.475115 T Mkwms B 58 345 OLLIMATEJD fol/Rae 5am h ecs/ven R FIELD FVIEW APPARATUS FOR SCANNING AN UNDERWATER AREA STATEMENT OF GOVERNMENTINTEREST The invention described herein may be manufactured and used byor for the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION There are spot scanning systems in the priorart mounted on airplanes, but these systems are incapable of mapping anunderwater target area. In the line scanning prior art methods, everypart of the line has the same intensity, for all practical purposes,however, in underwater scanning, this is not true because of the rapidattenuation of a light beam in water. A point in the middle of any linewould reflect light of much greater intensity than light reflected fromeither end of the same line, since the distance traveled by the incidentbeam of light to the middle of each line is considerably less than tothe ends of the same line, and hence the light intensity is attenuatedless in the middle of the line. Since light attenuates exponentiallyunderwater, this factor must be taken into consideration in any systemused for underwater viewing.

SUMMARY OF THE INVENTION This invention relates to an apparatus whichoptically maps underwater surfaces, displays the mapped image in realtime on a cathode ray tube, or stores the image in a video taperecorder. In operation the device is attached to either a towed orself-propelled underwater vehicle.

The apparatus includes the following basic components:

1. a point light source which is spectrally optimized for underwatertransmission;

2. collecting and collimating optics for the light source;

3. a dual prism and lenses mounted in a rotating cylinder;

4. receiving optics, for receiving the beam of light reflected from thetarget;

5. a photomultiplier tube, for converting the received beam of lightinto an electrical signal;

6. automatic gain control electronics, to compensate for the variableattenuation of the incident and reflected light beams;

7. a display cathode ray tube; or

8. a magnetic tape recorder, to display or record the target area;

9. a power supply, to supply energy where needed;

and

10. underwater housings.

OBJECTS OF THE INVENTION An object of the invention is to provide anapparatus for scanning an underwater area which may be used either on amoving vehicle or on a stationary platform.

Another object of the invention is to provide an apparatus for scanningan underwater target area which compensates for the differences inlength of the beam path at various parts of the linear scan.

Yet another object of the invention is to provide an apparatus which maybe used for night time surveillance of a target area.

III

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of theinvention, when considered in conjunction with the accompanyingdrawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially cross-sectional,partially diagrammatic view of the apparatus for scanning an underwaterarea.

FIG. 2 is a cross-sectional end view of the apparatus.

FIG. 3 is a diagrammatic view of the scanning pattern over the targetarea.

FIG. 4 is a cross-sectional and schematic end view of the apparatusshowing a stepper motor for incrementally scanning an underwater area.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thisfigure shows an apparatus 10 for linearly scanning, point by point, atarget area (not shown), generally underwater, comprising a housing 12,having an interior cylindrical wall 12W, the cylindrical surfacedefining an axis 18, which has two slots, 14 and 16 through itperpendicular to the axis of the wall.

A rotating, cylindrical scanning assembly 20, comprises a drivencylinder 22 disposed within the housing, the cylinder being truncated ateach end at approximately 45 to a plane through the axis 18 of thecylinder, so that in this plane the cross section is trapezoidal. Theaxis of the driven cylinder 22 and that of the wall 12W are coincident.

Two transparent, right, cylindrical prisms, 24L and 24R, truncated atapproximately 45, have the same diameter and the same axis of rotation18 as the driven cylinder 22, each prism bonded by at least part of itselliptical face to the corresponding surface at each end of the drivencylinder. The prisms, 24L and 24R, of course need not be truncatedcylindrical prisms, but may be the ordinary prisms, which are truncatedrectangular solids.

The scanning assembly 20 further comprises a pair of lenses, 26L and26R, one mounted to each prism, 241.. and 24R, respectively, at thecylindrical surface thereof so that they rotate with the prisms, theprincipal axis of each of the lenses being'convergent toward each otherso that the axes may be made to intersect, through the slots, 14 and 16,in the housing, at some point in the plane of the drawing, the point ofintersection coinciding with the point of the target being scanned. Asshown, the left and right lenses, 261. and 26R, are bonded by a thintransparent cement, not shown, directly to the left and right prisms,24L and 24R. However, in another arrangement, the lenses 26L and 26Rcould be mounted in such a way as to be supported by, but separatedfrom, the prisms 24L and 24R.

Driving means, 28 in FIG. 2, for rotating the scanning assembly 20, andtherefore the lenses, 26L and 26R permit scanning aline of the targetarea at a time, the line being substantially perpendicular to the axis18 of the cylinder.

The period of rotation for the cylindrical assembly 20 IS where e is theangular resolution of the aparatus 10 in degrees. R (see FIG. 2) is theminimum one-way range of the device, and v is the speed of the vehicle.Typical values for these parameters are e 0.25, R 100 ft., and v 15ft/sec. The corresponding value for T is 0.029 sec.

The resolution of the apparatus 10 in line pairs is N Bole 210 linepairs where the half-angle field of view, also shown in FIG. 2, isassumed to be 525. The apparatus shown in FIG. 1 may further comprise alight source 32, for generating a beam of light along the axis 18, andtoward the internally reflecting surface of, one of the prisms 24L,thereby projecting a beam of collimated light 348 onto the point of thetarget being scanned.

A photodetector, for example, a photomultiplier tube 36, is adapted toreceive the beam of light 34R reflected from the scanned point, which isfirst received by the right lens 26R, and then passes through the otherprisms 24R.

Two versions of employing the photodetector 36 may be embodied: (l) Thefield of view of the reflected beam of light 34R is stopped down at thephotodetector 36; or (2) the diameter of the beam of light 348 at thesource is made as small as possible.

The advantage in using a larger reflected beam of light 34R is that itameliorates the synchronization of aiming the reflecting lens 24R uponthe exact spot of the target which is being illuminated and no other. Ifthe returning beam of light 34R is stopped down too much, then asynchronization problem arises, namely, synchronization of the incidentbeam of light 345 aimed at the target, and the aiming of the reflecting,or receiving, lens 26R so that it is aimed at precisely the same point.

If the apparatus 10 is to be used underwater, it would further comprisean automatic gain control circuit (not shown), whose input is connectedto the output of the photodetector 36, to compensate for the variationsin the intensity of the received light 34R due to the varying length ofthe scanning and received beams of light at different parts of the line,52 in FIG. 3, of the target being scanned.

If the scanning angle, 20,, in FIG. 2, is relatively small so that thehypotenuse 42 has a length approximately the same as that of theperpendicular R, then an automatic gain control circuit is not required.

A calculated gain control could be used on the output signal from thephotomultiplier tube. For a Lambertian surface the ratio of the lightintensity incident on the receiver is ratio exp 25R lcos0/cos0)} cos 0,

when comparing the light reflected from a target element at the angle 0with that from the center of the field of view, as shown in FIG. 2.

A Lambertian surface is a standard diffuse surface such that lightreflected off it has the same intensity in all directions. The value ofthe parameter 5, would be adjusted so that the output signal of the gaincontrol devices would have a constant direct-current level. The value ofH, the effective attenuation coefficient, could also be recorded on adifferent track of the video tape so that a record of the opticalproperties of the water could be obtained simultaneously with a map ofthe surfaces optical reflectivity.

As is shown in FIG. 1, the apparatus 10 may further comprise a videotape recorder 38 whose input is connected to the output of the automaticgain control circuit, if present, for recording the target area on videotape.

In addition, the apparatus may further comprise a power supply 39, tofurnish energy for the driving means 28, light source 32, automatic gaincontrol circuit, and the video tape recorder 38.

The apparatus 10 may be used in a manner wherein the housing 12 isadapted for mounting to a moving vehicle, a line (52 in FIG. 3) of thetarget area being scanned at a time as the vehicle moves forward.

On the other hand, the apparatus may further comprise a stepper motor,62 in FIG. 4, for incrementally tilting the prisms, 24L and 24R, in adirection substantially at right angles to the scanning lines, 52 inFIG. 3, an increment being equal to the distance d between two adjacentscanning lines, thereby permitting scanning of an area to beaccomplished from a stationary platform. The duty cycle for theapparatus 10 with a rotating cylinder is duty cycle 20 /360 where 6,, isthe half-angle field of view, as shown in FIG. 2. The duty cycle for theapparatus 10 with stepper motors would be nearly one.

Instead of making complete revolutions, the apparatus 10 may include ascanning assembly 20 wherein the rotation is less than 360, so that thescanning assembly oscillates back and forth, the included angle ofoscillation being sufficient to encompass the width of the target area.This type of embodiment would include a stepper motor, 62 in FIG. 4,which could also be used to change the pitch of the source prism withinthe rotating cylinder, causing the oscillation to progress in smallincremental steps, rather than in one continuous oscillation. Thisallows the collimated source beam to 348 be scanned longitudinally, andhence the apparatus 10 could be used as a stationary volume-scanningimaging system. It is very desirable that the stepper motor be able tomake the increments of the steps as small as the optical resolution 54,in FIG. 3, of the apparatus 10.

In another embodiment of an apparatus for linearly scanning a targetarea, instead of the pair of prisms, 24L and 24R, a pair of mirrors, oneattached to each end of the cylinder 22 are used. The pair of mirrorsare mounted to the driven cylinder 22 in a manner so that they rotatewith the cylinder. The light source 32 generates a beam of light alongthe axis 18 of the cylinder, and toward the reflecting surface of one ofthe mirrors, thereby projecting a beam of light 34R through one of thelenses 26L and onto the point being scanned. A photodetector receivesthe beam of light 34R reflected from the scanned point, which is firstreceived by the other lens 26R and then passes to the other mirror.

The automatic gain control circuit serves the same function as in theembodiment 10 described in detail hereinabove.

Instead of the video tape recorder 38, of the apparatus 10 shown in FIG.1, the second embodiment includes a video display, including acathode-ray tube (CRT), whose input is connected to the output of theautomatic gain control circuit, for displaying the target area on thescreen of the CRT.

When using a video display, including a cathode-ray tube, a videosignal, which is the output of the photomultiplier, forms the input tothe video display. The scanning of the video signal is done by the sameelectronics that causes scanning of the spot of the cathoderay tube. Thehorizontal and vertical synchronization rates may be determined, asindicated hereinbelow, from the required vertical rate of scan, which isrelated to the speed of the vehicle on which the apparatus is mounted,and the designed horizontal resolution of the apparatus. The scan of theelectron beam of the CRT is synchronized with the optical scan of thelight beam.

The cathode-ray tube would either have to be a longpersistence type orsome means of storage of the video information would have to beprovided, so that most of the video information would remain on thescreen and only new information would have to be entered.

Essentially, when used on a moving vehicle, all lines 52 of the imagewould move upwardly one line at a time, and only a new, bottom, linewould be entered onto the prior image. Thus, the image seen on the CRTscreen would correspond to the actual image under the moving vehicle atall times. The image on the CRT screen would move across the screen atthe same rate as the apparent movement of the target area.

In yet another embodiment, an oscillating cylinder driven by aneccentric on a rotating gear could be used to azimuthally direct thelight source and receiving 0ptics. The duty cycle for this device wouldbe nearly one. Also the varying angular speed of the cylinder wouldsomewhat compensate for the non-uniform transverse scan speed across thesurface of the object. The transverse scan speed is related to theangular speed by (ds/dt) (d/dt) (R tan0) (R/cos 0 (dG/dt).

A further refinement of the apparatus for scanning an underwater targetarea involves optical signal processing to improve the image quality.Because of the finite size of the scanning point, the picture qualitywill be somewhat blurred. The picture quality may be improved by spatialfiltering. This will enable sharpening up the contrast of the image atthe edges. It would crispen the quality of the picture, rather thanleaving it subdued.

The advantages and new features of the apparatus of the invention are:

a. The apparatus 10 effectively utilizes the method of volume scanningto eliminate backscattered light.

b. The rotating cylindrical assembly insures the azimuthalsynchronization of the light source and receiver.

0. The adjustable receiver field stop insures range synchronization ofthe light source 32 and the receiving optics.

d. The apparatus 10 has tremendous source gain over a conventional wideangle viewing system by concentrating all of the power emitted by thelight source 32 in a nearly collimated beam 34S. If the nearlycollimated beam 348 has a half angle of 0.5, while the half angle of thewide angle viewing system is 52.5, the corresponding source gain of thesystem is S0 10 log l cos(52.5)/l cos(0.5)}= db e. The apparatus 10utilizes the high sensitivity and large dynamic range of thephotomultiplier tube 36 as the receiver element in an underwater mappingsystem.

. 6 f. The automatic gain control on the output signal of thephotomultiplier 36 compensates for varying propagation losses which areinherent in a wide angle imaging or mapping system which operates in anabsorbing medium.

g. The apparatus 10 can obtain a continuous record of the opticalproperties of the water.

h. The apparatus It) operates in real time.

e. The cost of the apparatus 10 appears to be less than alternativemethods.

Although basically the invention is designed to be used for mapping anunderwater target, the apparatus may also be used for mapping aboveground terrain, using a laser beam as a light source. For such use, theautomatic gain control circuit would not be required.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. An apparatus for linearly scanning, point by point, a target area,generally underwater, comprising:

a housing, having an interior cylindrical wall, the cylindrical surfacedefining an axis, the housing having two slots through it perpendicularto the axis of the wall;

a rotating, cylindrical scanning assembly, disposed within thecylindrical wall of the housing, comprising:

a driven cylinder disposed within the housing, the

cylinder being truncated at each end at approximately 45 to a planethrough the axis of the cylinder, so that in this plane the crosssection is trapezoidal;

two transparent, right, cylindrical prisms, truncated at approximately45, having the same diameter and the same axis of rotation as the drivencylinder, each prism bonded by at least part of its elliptical face tothe corresponding surface at each end of the driven cylinder; and

a pair of lenses, one mounted to each prism at the cylindrical surfacethereof so that they rotate with the prisms, the principal axis of eachof the lenses being convergent toward each other so that the axes may bemade to intersect, through the slots in the housing, at some point insaid plane, the point of intersection coinciding with the point of thetarget being scanned; and

driving means for rotating the scanning assembly, and therefore thelenses, thereby scanning a line of the target area at a time, the linebeing substantially perpendicular to the axis of the cylinder.

2. The apparatus according to claim 1, further comprising:

a light source, for generating a beam of light along the axis of, andtoward the internally reflecting surface of, one of the prisms, therebyprojecting a beam of light through one of the lenses and onto the pointbeing scanned; and

a photodetector, adapted to receive a beam of light reflected from thescanned point, which is first received by the other lens and then passesthrough the other prism.

3. The apparatus according to claim 2, further comprising:

an automatic gain control circuit, whose input is connected to theoutput of the photodetector, to compensate for the variations in thereceived light intensity due to the varying length of the scanning andreceived beams of light at different parts of the line of the targetbeing scanned.

4. The apparatus according to claim 3, further comprising:

a video tape recorder, whose input is connected to the output of theautomatic gain control circuit, for recording the target area on videotape.

5. The apparatus according to claim 4, further comprising:

a power supply, to furnish energy for the driving means, light source,automatic gain control circuit, and the video tape recorder.

6. The apparatus according to claim 5, wherein the housing is adaptedfor mounting to a moving vehicle, a line of the target area beingscanned at a time as the vehicle moves forward.

7. The apparatus according to claim 5, further comprising:

a stepper motor for incrementally tilting the prisms in a directionsubstantially at right angles to the scanning lines, an increment beingequal to the distance between two adjacent scanning lines, permittingscanning of an area to be accomplished from a stationary platform.

8. The apparatus according to claim 1, wherein the rotation is less than360, so that the scanning assembly oscillates back and forth, theincluded angle of oscillation being sufficient to encompass the width ofthe target area.

9. The apparatus according to claim 8, wherein the driving means forrotating the scanning assembly comprises:

a stepper motor, which causes the oscillation to progress in smallincremental steps, rather than in one continuous oscillation.

10. An apparatus for linearly scanning, point by point, a target area,generally underwater,comprising:

a housing, having an interior cylindrical wall, the cylindrical surfacedefining an axis, the wall having two slots through it perpendicular tothe axis of the wall;

a rotating, cylindrical scanning assembly, disposed within thecylindrical wall of the housing, comprising: a driven cylinder disposedwithin the housing, the cylinder being truncated at each end atapproximately 45 to a plane through the axis of the cylinder, so that inthis plane the cross section is trapezoidal;

a pair of mirrors, one attached to each end of the cylinder;

a pair of lenses mounted to the cylinder in a manner so that they rotatewith the cylinder, the principal axis of each of the lenses beingconvergent toward each other so that the axes may be made to intersect,through the slots in the housing, at some point in said plane, the pointof intersection coinciding with the point of the target being scanned;

driving means for rotating the scanning assembly, and therefore thelenses, thereby scanning a line of the target area at a time, the linebeing substantially perpendicular to the axis of the cylinder;

a light source, for generating a beam of light along the axis of, andtoward the reflecting surface of, one of the mirrors thereby projectinga beam of light through one of the lenses and onto the point beingscanned; and

a photodetector, adapted to receive a beam of light reflected from thescanned point, which is first received by the other lens and then passesthrough the other mirror;

an automatic gain control circuit, whose input is connected to theoutput of the photodetector, to compensate for the variations in thereceived light intensity due to the varying length of the scanning andreceived beams of light at different parts of the line of the targetbeing scanned;

a video display, including a cathode-ray tube, whose input is connectedto the output of the automatic gain control circuit, for displaying thetarget area; and

a power supply, to furnish energy for the driving means, light source,automatic gain control circuit,

and the video display.

1. An apparatus for linearly scanning, point by point, a target area,generally underwater, comprising: a housing, having an interiorcylindrical wall, the cylindrical surface defining an axis, the housinghaving two slots through it perpendicular to the axis of the wall; arotating, cylindrical scanning assembly, disposed within the cylindricalwall of the housing, comprising: a driven cylinder disposed within thehousing, the cylinder being truncated at each end at approximately 45*to a plane through the axis of the cylinder, so that in this plane thecross section is trapezoidal; two transparent, right, cylindricalprisms, truncated at approximately 45*, having the same diameter and thesame axis of rotation as the driven cyliNder, each prism bonded by atleast part of its elliptical face to the corresponding surface at eachend of the driven cylinder; and a pair of lenses, one mounted to eachprism at the cylindrical surface thereof so that they rotate with theprisms, the principal axis of each of the lenses being convergent towardeach other so that the axes may be made to intersect, through the slotsin the housing, at some point in said plane, the point of intersectioncoinciding with the point of the target being scanned; and driving meansfor rotating the scanning assembly, and therefore the lenses, therebyscanning a line of the target area at a time, the line beingsubstantially perpendicular to the axis of the cylinder.
 2. Theapparatus according to claim 1, further comprising: a light source, forgenerating a beam of light along the axis of, and toward the internallyreflecting surface of, one of the prisms, thereby projecting a beam oflight through one of the lenses and onto the point being scanned; and aphotodetector, adapted to receive a beam of light reflected from thescanned point, which is first received by the other lens and then passesthrough the other prism.
 3. The apparatus according to claim 2, furthercomprising: an automatic gain control circuit, whose input is connectedto the output of the photodetector, to compensate for the variations inthe received light intensity due to the varying length of the scanningand received beams of light at different parts of the line of the targetbeing scanned.
 4. The apparatus according to claim 3, furthercomprising: a video tape recorder, whose input is connected to theoutput of the automatic gain control circuit, for recording the targetarea on video tape.
 5. The apparatus according to claim 4, furthercomprising: a power supply, to furnish energy for the driving means,light source, automatic gain control circuit, and the video taperecorder.
 6. The apparatus according to claim 5, wherein the housing isadapted for mounting to a moving vehicle, a line of the target areabeing scanned at a time as the vehicle moves forward.
 7. The apparatusaccording to claim 5, further comprising: a stepper motor forincrementally tilting the prisms in a direction substantially at rightangles to the scanning lines, an increment being equal to the distancebetween two adjacent scanning lines, permitting scanning of an area tobe accomplished from a stationary platform.
 8. The apparatus accordingto claim 1, wherein the rotation is less than 360*, so that the scanningassembly oscillates back and forth, the included angle of oscillationbeing sufficient to encompass the width of the target area.
 9. Theapparatus according to claim 8, wherein the driving means for rotatingthe scanning assembly comprises: a stepper motor, which causes theoscillation to progress in small incremental steps, rather than in onecontinuous oscillation.
 10. An apparatus for linearly scanning, point bypoint, a target area, generally underwater,comprising: a housing, havingan interior cylindrical wall, the cylindrical surface defining an axis,the wall having two slots through it perpendicular to the axis of thewall; a rotating, cylindrical scanning assembly, disposed within thecylindrical wall of the housing, comprising: a driven cylinder disposedwithin the housing, the cylinder being truncated at each end atapproximately 45* to a plane through the axis of the cylinder, so thatin this plane the cross section is trapezoidal; a pair of mirrors, oneattached to each end of the cylinder; a pair of lenses mounted to thecylinder in a manner so that they rotate with the cylinder, theprincipal axis of each of the lenses being convergent toward each otherso that the axes may be made to intersect, through the slots in thehousing, at some point in said plane, the point of intersectioncoinciding with the point of the target being scanned; driving means fOrrotating the scanning assembly, and therefore the lenses, therebyscanning a line of the target area at a time, the line beingsubstantially perpendicular to the axis of the cylinder; a light source,for generating a beam of light along the axis of, and toward thereflecting surface of, one of the mirrors thereby projecting a beam oflight through one of the lenses and onto the point being scanned; and aphotodetector, adapted to receive a beam of light reflected from thescanned point, which is first received by the other lens and then passesthrough the other mirror; an automatic gain control circuit, whose inputis connected to the output of the photodetector, to compensate for thevariations in the received light intensity due to the varying length ofthe scanning and received beams of light at different parts of the lineof the target being scanned; a video display, including a cathode-raytube, whose input is connected to the output of the automatic gaincontrol circuit, for displaying the target area; and a power supply, tofurnish energy for the driving means, light source, automatic gaincontrol circuit, and the video display.